1. Field of the Invention
This invention relates to solar energy grade cadmium sulfide. More particularly, it relates to cadmium sulfide having advantageous particle size, bulk density and purity to enable the material to be effectively used for solar energy devices and to a process for producing the improved cadmium sulfide.
2. Prior Art
Cadmium sulfide is known in the art and photovoltaic cells made therefrom can have an efficiency that compares favorably with silicon. There has been a problem heretofore in obtaining a material that can be deposited satisfactorily upon a suitable substrate to achieve good adherence and a high quality. Cadmium sulfide, when of satisfactory purity, has been very finely divided, e.g., below 0.5 micrometers with an average agglomerated size of about 2 micrometers. This fine aggregate had to be pelletized before it could be satisfactorily deposited by sublimation upon a substrate. Normally vacuum systems are used and as a consequence the finely divided material before pelletization was completely unsatisfactory. Furthermore, prior art cadmium sulfide contained a relatively high concentration of volatile impurities of organic and inorganic origin. The impurities result in lower deposition efficiency, poor adhesion (bonding) and lower conversion efficiency as well as contributing to maintenance problems in the vacuum processing equipment.
A free-flowing cadmium sulfide material having an average particle size of from about 8 to about 25 micrometers and having less than about 1% of the particles below about 5 micrometers and having a low level of anionic, cationic and volatile impurities would be an advancement in the art.